1. Field of the Invention
The present invention relates to a heat insulating structure for an expansion turbine that is provided in a helium refrigerator or the like, and a method of manufacturing the heat insulating structure.
Priority is claimed on Japanese Patent Application No. 2007-089023, filed on Mar. 29, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
The following adiabatic expansion device is known as one kind of expansion turbine (for example, see Japanese Patent Application, First Publication No. 6-137101 and Japanese Patent Application, First Publication No. 2001-132410). The adiabatic expansion device includes an expander body that includes an outlet passage for a refrigerant fluid at a central portion thereof and an introduction chamber for the refrigerant fluid communicating with an inlet of the outlet passage on an outer peripheral portion thereof, and a turbine impeller that is rotatably provided at the inlet of the outlet passage and braked by a braking device. The adiabatic expansion device adiabatically expands the refrigerant fluid, such as helium by rotating the turbine impeller with the refrigerant fluid that has ultra low temperature and flows from the introduction chamber toward the outlet passage. Then, the adiabatic expansion device discharges the refrigerant fluid, the temperature of which falls, through an outlet of the outlet passage.
However, in the expansion turbine in the related art, the introduction chamber and the outlet passage of the expander body are isolated from each other via a solid partition wall that surrounds the entire periphery of the outlet passage. For this reason, during the operation of the expansion turbine, the heat of the refrigerant fluid corresponding to a high temperature side, which is introduced into the introduction chamber, is transferred to the refrigerant fluid corresponding to a low temperature side, which flows in the outlet passage, through the partition wall. Therefore, there is a problem in that turbine performance deteriorates. When the difference between the inlet and outlet temperatures of the refrigerant fluid in the expansion turbine is large, this problem occurs much more significantly. However, appropriate measures against the problem have not been provided yet.